I run 44-46mm tires. Ive been messing with the 189 with my labtop but when i run boost at max it doesnt seem any faster. Our track has alot of turns in it so there isnt much straight speed. I am going again today to keep messing with my setup. Anymore suggestions? Not sure if this helps, but the track is Debbiesrcworld in Chesapeake, VA indoor carpet.